Sea-going vessels such as ships, submarines or the like may be required to deploy and retrieve cables including, for example, but not exclusively, towed arrays, sonar equipment or the like. Deployment or retrieval of such cables may be facilitated by use of a capstan or similar device.
Referring to FIG. 1 of the drawings, a single drum capstan arrangement is shown, the capstan comprising a single capstan drum or spindle 10 around which a cable or rope 12 may be wrapped or wound. An alternative arrangement (not shown) provides a double drum capstan comprising two multi-groove drums, one drum having one less groove than the other. In use, a cable is adapted to be wound alternately around the first and second drums.
A capstan, whether comprising a single or double drum, may be utilised as a means of achieving a mechanical advantage in the addition or reduction of tension during deployment or retrieval of the cable. It will be recognised that the amount of tension that may be removed or added by wrapping around a capstan is a function of: the initial tension (T1); the co-efficient of friction (μ) between the capstan and the cable; and the angle of contact (β radians) between the capstan and the cable. Thus, the tension may be ascertained from the capstan equation, T2=T1eμβ.
For example, in the capstan arrangement shown schematically in FIG. 1, where the co-efficient of friction (μ) between the cable and capstan drum is 0.10, the tension ratio (T2\T1) of a single groove is approximately 1.369 for an angle of contact of approximately 180 degrees (3.14 radians). This is based on the assumption of infinite rigidity of the drum and cable. However, in reality, whereas drums tend to be relatively rigid, cables, ropes and, in particular, sensor arrays may not be.
It will be recognised that, when a capstan is engaged in retrieving a cable experiencing hydrodynamic drag, as the cable progresses from one groove to the next groove, the tension in the cable reduces. Thus, the cable will contract and its linear velocity will reduce. However, the surface velocity of the drum will remain constant. It is evident that where a difference in velocity is present, distortion between the capstan drum and the cable may arise. Therefore, relative elasticity between the drum and the cable may introduce relative velocities, or slippage, this slippage between the cable and drum resulting in frictional stresses which can be damaging to the cable.
Cable damage is of particular concern where it relates to an expensive and sophisticated cable such as a submarine or surface ship acoustic array and the distortion and/or damage to the cable caused by the relative velocity, or slippage, may result in the operational effectiveness of the vessel being compromised.